Various mechanical apparatus include first and second members, which are substantially coaxial and disposed for relative reciprocal movement. Exemplary are internal combustion engines, positive displacement pumps, pneumatic motors and other mechanical devices incorporating a piston and a cylinder assembly. Other examples will occur to the skilled artisan.
Commonly, the cylinder is defined by a bore having a cylindrical sidewall. The piston, a generally cylindrical member, includes a sidewall and a top. An end wall or head, opposing the top of the piston, closes one end of the bore. A chamber of variable capacity is defined within the bore between the head and the top of the piston. Pressurized fluid functions in this chamber. In an internal combustion engines, the fluid is the expanding gas of combustion which ignites and propels the piston downwardly or away from the head. Pressurized fluid from an external source is introduced into the chamber of a pneumatic motor to force movement of the piston. In a pump, which is companion to the engine and to the pneumatic motor, fluid is compressed and pressurized in the chamber.
To provide for relative movement between the members, the diameter of the piston must be less than the diameter of the bore. Frequently, substantial clearance is required. For example, it is well known that a piston is more sensitive to thermal expansion than is a cylinder. Accordingly, where heat is a factor, such as in internal combustion engine, additional space must be provided between the sidewall of the bore and the sidewall of the piston to accommodate thermal expansion of the piston. The space, commonly referred to as sidewall clearance, is generally in the range of fifteen ten-thousandths of an inch to five one-thousandths of an inch, or greater.
However, the piston must be sealed to the cylinder. Conventionally, this is accomplished by a device commonly referred to as a piston ring, an annular seal usually fabricated of metal such as cast iron. The piston ring is received within a mating annular ring groove formed into the sidewall of the piston. To facilitate expansion during installation, and for other reasons, the piston ring is radially severed.
The spacing between the opposed ends of the severed piston ring, known as the end gap, serves various functions after installation. Having an inherent tendency to expand, the ring maintains constant tension for attendant sealing against the sidewall of the cylinder as the bore increases in diameter as a result of wear. The end gap also allows for thermal expansion of the piston ring resulting from heat generated by friction, compression of fluid, combustion of fuel and other sources.
Efficiency, economy and service life of the apparatus is directly related to blow-by. In general, less than optimum output of the apparatus results from loss of pressure or compression of the fluid. Other deleterious effects are unique to the particular apparatus. In an internal combustion engine, for example, contaminating by-products of combustion suspended in the blow-by gases are carried into the lubricating system, which harms components throughout the engine and produces an attendant power loss. In internal combustion engines, piston rings that fail to seal the pistons to the cylinders that result in the attendant blow-by can reduce the engine's power by up to forty percent depending on the engine's displacement, compression ratio and speed.
In recognition of the desirability of enhancing the seal between the piston and the cylinder, the prior art has proposed various seals, which purportedly reduce or eliminate blow-by. Several prior art proposals are direct attempts to eliminate the end gap in the conventional piston ring. Various proposals include an insert, which spans the end gap and is received in a notch formed into the ring on either side of the end gap. Other proposals include the use of a relatively thin steel member, alternately named a ribbon member or a gap seal member, having a substantially rectangular cross-section. Also advanced is a plurality of severed annular members, installed in stacked arrangement with staggered end gaps. The prior art has also advocated the use of thin steel members, colloquially dubbed rails, in combination with ring members having a general resemblance to conventional piston rings. Being of substantially heavier construction than a rail and usually fabricated of malleable material, such as cast iron, the ring member is variously referred to as a packing member or a sealing ring. The ring member in combination with the rail member forms a seal assembly of which various embodiments are known.
For various reasons, annular seals of the foregoing character are not entirely satisfactory. For example, in an assembly wherein the rail resides within the seal member and exerts an expansive force, excessive friction is generated against the sidewall of the bore. Where only the rail contacts the sidewall, seating or breaking-in of the seal is substantially retarded or even prohibited. An angled rail is subject to vibration, commonly known as ring flutter, when used in connection with a rapidly reciprocating piston. Seal assemblies of the foregoing type tend to be inherently heavy and highly tensioned so as to be prone to wear and prone to produce cylinder distortion.
Efforts to improve upon piston rings by eliminating the end gap for the purpose of eliminating or reducing blow-by disregard are more fundamental problem with known annular seals. Of particular significance is the inherent tendency of expansion of the annular seal to maintain tension for attendant sealing against the sidewall of the bore. The tension exerted against the sidewall of the bore is often unnecessarily high, which results in unwanted and premature wear than can compromise the seal leading to power loss and fuel inefficiency in internal combustion engines. The tension exerted against the sidewall of the bore is also uneven, which leads to prolonged wear-in and seating. Even after seating, the irregular tension exerted by the annular seal against the sidewall of the bore persists, which results in the irregular wear of the annular seal and the sidewall of the bore, which can produce power loss and poor fuel economy.
Of additional concern is piston ring tension. Piston ring tension is characterized by tangential tension, the amount of force needed to squeeze the ends of the ring together, and unit tension or unit pressure, the amount of pressure exerted by the contact surface of face of the ring against the cylinder wall. In the 1970s, conventional piston rings had tangential tensions of up to thirty pounds, with from 22 to 26 pounds being standard. Low compression rings, rated at from five to twelve pounds, are used in most engines today. Most aftermarket low tension rings have a somewhat higher tension than the original equipment (OE) rings they replace. For example, if an OE ring specification calls for 6 to 12 pounds, an aftermarket ring can have as much as 12 to 16 pounds. Higher tension is needed because rings are often installed in oversized cylinders. Cylinder bores can also have more distortion than a new engine, so extra loading improves sealing.
Low tension piston rings reduce friction, improving fuel economy and cylinder sealing. The amount of force the ring exerts against the cylinder wall, unit pressure or unit tension, depends on tangential tension as well as ring thickness and cylinder bore diameter. Low tension rings are inherently thinner and exert less pressure against cylinder walls than conventional rings. Low tension piston rings, being thinner than standard piston rings, unfortunately tend to become distorted when exposed to extreme engine heat, which can compromise the seal with the cylinder resulting in blow-by, loss of power, and poor fuel economy.
Accordingly, there is a need for an improved piston ring that minimizes fluid leakage between reciprocally movable members, that is selectively tensioned to reduce wear to the piston ring and the bore sidewall, that is selectively tensioned to exert even pressure against the bore sidewall, and that tends to resist distortion when exposed to extreme engine heat.